We are studying the effects of diethylstilbesterol on phosphatidylcholine biosynthesis in rooster liver as a model system for hormonal control of phospholipid biosynthesis. Injection of this steroid (20 mg/kg body weight) results in a large increase in serum phospholipids and an enhanced incorporation of 3H-choline into phosphatidylcoline. The studies should pinpoint which reaction in phosphatidylcholine biosynthesis is stimulated by hormone action. Subsequent studies will be directed to how the hormone causes the reaction to be stimulated. Current evidence suggests that the reaction catalyzed by CTP:phosphocholine cytidylyltransferase is stimulated by the hormone treatment. The mechanism for the effect is presently unknown. The results should lead to a better understanding of the control of phosphatidylcholine biosynthesis and the sectetion of phospholipids and lipoporotiens by liver.